With recent increases in the packing density of semiconductor devices, conventional aluminum wiring material must be replaced with copper to reduce signal retardation and to improve electromigration resistance. However, when copper is used as a wiring material, the copper is diffused into an interlayer insulating film (for example, a silicon oxide layer) which defines the wiring. To solve this problem, a diffusion barrier layer (formed on the side wall and the bottom of copper wiring) and a capping layer (formed on the upper part of copper wiring) are formed between copper wires, and an interlayer insulating film is applied to prevent direct contact.
A silicon nitride layer has been used up to now as a capping layer for copper wiring. However, in addition to poor adhesion to copper, the silicon nitride layer has a different thermal expansion coefficient to the interlayer insulating film formed on the upper part of the capping layer, for example a silicon oxide layer, so mechanical stress is concentrated between the capping layer and the interlayer insulating film, resulting in the separation of the capping layer (silicon nitride film) from the upper part of the copper wiring. When the capping layer is separated from the copper wiring, the diffusion of copper into the interlayer insulating film will not be inhibited. Owing to the huge difference in permittivity of the silicon nitride film, the volume of parasitic capacitance increases, which causes retardation of the driving speed of a semiconductor device by RC retardation.
Thus, a cobalt-based alloy has been proposed as an alternative, which seems to have excellent adhesion to copper wiring and low permittivity and prevents the diffusion of copper into an interlayer insulating film. The cobalt-based alloy contains cobalt as a major component and additionally includes metals such as tungsten, boron, phosphorus, etc. To form a cobalt-based alloy thin film selectively on the upper part of copper wiring, electroless plating has been proposed.
Electroless plating is a method of forming a metal thin-film by reducing a metal ion using an electron generated through oxidation of a reducing agent on the surface of a catalyst substrate without any externally supplied electrons. This method has an advantage of forming a metal thin-film specifically on a target area, which is activated by catalyst, of an entire substrate. However, by containing a reducing agent the plating solution becomes unstable, according to the plating conditions, and thus autolysis occurs. Autolysis indicates that a metal ion is reduced not on the surface of a catalyst substrate, but in the plating solution, thus forming a precipitate. This autolysis brings about the loss of metal particles, resulting in a decrease in the durability of the solution (shortening the life of the solution) and a decrease in the quality of the metal thin-film due to the formation of a precipitate in the solution.
To apply a cobalt-based alloy on the upper part of copper wiring by electroless plating, dimethylamine borane (DMAB), which is easily oxidized on the surface of copper, has to be used as a reducing agent considering the low catalytic activity of copper, and the temperature for the process has to be high. However, in this case the electroless plating solution becomes chemically unstable, so autolysis occurs easily.